Add preliminary support for Mac kern table.
Add preliminary support for Mac kern table.

This is to support fonts like "Anton" which only have a Mac kern table.

axkibe/opentype.js,fpirsch/opentype.js,nodebox/opentype.js,moyogo/opentype.js,moyogo/opentype.js,Jolg42/opentype.js,brawer/opentype.js,googlefonts/opentype.js,fpirsch/opentype.js,axkibe/opentype.js,Jolg42/opentype.js,nodebox/opentype.js,googlefonts/opentype.js,brawer/opentype.js